This invention relates to a method of making a two-phase material comprising small fibers in concrete as in U.S. Pat. No. 3,429,094 of James P. Romualdi. It has to do particularly with improvements for providing such material with at least a preselected flexural strength.
For years, the low tensile strength of concrete has been recognized and has been accepted as an unavoidable limitation to be taken into account in considering applications for concrete. In the past, attempts to compensate for the lack of tensile strength in concrete have usually involved either unstressed or prestressed reinforcing steel rods. Neither really overcomes the low tensile strength limitation, but each merely bypasses it. Thus, concrete reinforced in this conventional manner is characterized by the fact that cracks form and propagate through regions in tension. Such cracks form at relatively low stresses and continue either until they reach a free surface or until they reach a region that is in compression.
Furthermore, the tensile strength exhibited in conventionally reinforced or plain concrete, as determined by standard tests, cannot be relied upon for design purposes since the formation of small incipient cracks due to fatigue, thermal shock or cavities substantially reduces the tensile strength of the concrete. Incipient formation of cracks and subsequent propagation thereof is not always evident, but the conditions favorable for the same cannot be predicted. Even the exercise of exceptionally rigid control procedures does not always minimize crack formation and growth. It has, therefore, long been accepted that about one-half of the material in a normally reinforced concrete beam is useless to resist tensile loads. The detrimental effects of tension cracks in reinforced concrete beams are not limited solely to loss of load-resistant area. For example, it is apparent that tension cracking severely limits the effectiveness of concrete tanks for containing liquids. Also, the use of reinforced concrete in marine structures is sharply curtailed because of the corrosive effect of salt water on reinforcing steel exposed by tension cracks.
The Romualdi invention provides a two-phase material comprising concrete and fine short fibers therein having first crack tensile strength in the order of two to three times that of conventionally reinforced concrete and characterized by substantial resistance to the formation and propogation of tensile or fatigue cracks. The material possesses attributes of an homogeneous material in that it experiences extensive plastic flow without disintegration. It also has the ability to absorb energy more efficiently than conventionally reinforced concrete.
The present invention primarily comprises improved methods of providing at least a preselected flexural strength in two-phase material of the Romualdi type by including and distributing fibers in a concrete mix in specific sizes and quantities to provide certain average bond areas of the fibers intersecting planes normal to the stress at known regions of highest tensile stress. It has been found as a part of the present invention that the first crack flexural strength and the ultimate flexural strength in such materials depend upon such average bond areas and that substantial improvements are obtained by appropriately controlling the sizes and quantities of the fibers as hereinafter disclosed. The bond area of each fiber is approximately equal to the perimeter of its cross section multiplied by its length.